Technical Field of the Invention
The invention is in the field of controlling temperature fluctuations in periodic gas flows. One specific application is in the field of breathing apparatus, such as are used by firefighters or miners.
Description of the Related Art
One situation where cooling of periodic air flow is required is for wildland firefighters in fire shelters. When trapped by a wildfire, firefighters can use a portable metallized shelter that provides some protection from the extreme temperatures and toxic gases generated in the event of a burnover. Entrapment conditions caused by a wildfire, however, are often harsh enough to lift the shelter's edges or tear the shelter, letting smoke and superheated air inside. Should the fire shelter become compromised, the shelter does little to protect the firefighter from smoke, toxic gases (e.g., carbon monoxide (CO)), or superheated air, and the respiratory tract can be damaged very quickly. In order to avoid respiratory damage, inhaled air may need to be at a temperature of 48° C. (119° F.) or cooler.
Another situation in which cooling of respiratory air is useful is in rebreathing apparatus, such as those used by miners. Rebreathers are used in mine rescue, firefighting, and other applications where portable breathing air is needed for periods of time greater than can be provided by a standard open-circuit air pack (a self-contained breathing apparatus (SCBA)). In mine rescue, rebreathers may be used when reentering a mine after an accident to rescue stranded miners and/or perform other tasks necessary to bring the mine back online. Standard mine rescue rebreathers supply oxygen for 4 hours.
Another type of rebreather is a 10-30 minute self-contained, self-rescuer (SCSR) that is carried by individual miners or kept in close proximity so that in case of an emergency, the SCSR can be donned and the miner can escape to a safe location. All rebreathers conserve air (and therefore minimize size and weight) by recirculating exhaled air through a system that removes carbon dioxide (CO2) with an alkali scrubber (commonly soda-lime or lithium hydroxide), while metering oxygen into the return air from a small compressed gas cylinder to maintain a desired inhaled oxygen (O2) concentration, for example a concentration of at least 19.5%. Considerable heat is generated when CO2 is removed using alkaline scrubbers, and additional heat is generated when moisture in the exhaled breath is condensed. In full size (e.g., rescue size) commercial devices, this heat is removed using ice packs that are installed into the unit just prior to use. In the smaller SCSR, there is no room for an ice pack so the unit relies on its thermal mass to absorb the heat. Depending on the ambient conditions, when the SCSR is used, its mass may or may not be sufficient to keep the temperature of the inhaled air below the requirement of 48° C. (119° F.).